I. Field of the Invention
The present invention relates to commercial and institutional kitchen exhaust systems, and more particularly, to an energy conserving exhaust rate control method and apparatus for exhaust systems for such kitchens.
II. Description of the Prior Art
Commercial and institutional kitchens are equipped to prepare food for large numbers of people and may form part of or adjoin larger facilities such as restaurants, hospitals and the like. Such kitchens are typically equipped with one or more commercial duty cooking units capable of cooking large amounts of food. On such a scale, the cooking process may generate substantial amounts of heat and air-borne cooking by-products such as water vapor, grease particulates, smoke and aerosols, all of which must be exhausted from the kitchen so as not to foul the environment of the facility. To this end, large exhaust hoods are usually provided over the cooking units, with duct work connecting the hood to a motor driven exhaust fan located outside the facility such as on the roof or on the outside of an external wall. As the fan is rotated by the motor, air within the kitchen is drawn into the hood and exhausted to the outside atmosphere. In this way, heat and cooking by-products generated by the cooking units follow a flow path from the cooking units into the hood towards the duct so that they may be liminated from the kitchen before they escape into the kitchen environment and perhaps into the rest of the facility.
As is conventional, the motor driving the exhaust fan rotates at a fixed speed. The exhaust fan thus rotates at a fixed speed as well and, therefore, tends to draw a constant volume of air through the hood. However, the amount of heat and/or cooking by-products generated by the cooking units will vary widely over the course of the day. If the selected fan speed is too low, during peak cooking periods, the fan will underexhaust allowing heat and/or cooking by-products to escape from the hood and into the kitchen and, perhaps, the rest of the facility. Accordingly, it has been the practice to select a speed for the fan that will exhaust the heat and cooking by-products generated during anticipated peak usage of the cooking units. It if often the case, however, that peak generation of heat and cooking by-products only infrequently occurs. Under these conditions, the exhaust fan continues to draw a volume of air intended to pull maximum heat and cooking by-product from the kitchen even though it is unnecessary to draw that large a volume of air. This condition, known as overexhausting, is very energy inefficient. For example, if the exhaust fan motor is running continuously at a high speed, much of the time the motor is consuming energy unnecessarily. Similarly, the life of the exhaust fan motor may be shortened as a result.
Overexhausting also poses a source of substantial energy waste in connection with the heating, ventilation and air conditioning (HVAC) which is utilized to condition the air in the kitchen and/or the rest of the facility. Usually, the entire facility including the kitchen must be maintained with a humanly acceptable and comfortable internal atmospheric environment. This is normally accomplished with one or more HVAC systems to provide conditioned air which is heated or cooled, humidified or dehumidified, and/or recirculated or replenished with fresh air in accordance with the demands of the seasons and the use of the facility. Such HVAC systems consume large amounts of energy and contribute substantially to the cost of the facility's overall operating budget. Unfortunately, substantial volumes of conditioned air pass out of the facility through the kitchen exhaust along with the heat and cooking by-products generated by the kitchen cooking units. As a consequence, the HVAC must make up for the lost volume of conditioned air by conditioning more air, thus resulting in consumption of more energy by and further loading of the HVAC system.
A still further drawback to overexhausting is the negative pressure created by the exhaust fan. Such negative pressure created in the kitchen tends to draw air from the rest of the facility into the kitchen setting up a draft in the facility. Some conventional kitchen exhaust systems include make-up air fans which provide outside air into the kitchen in the environment of the hood in an effort to balance pressure between the kitchen and the rest of the facility. Provision for make-up air also helps to reduce the amount of conditioned air which is lost to the kitchen exhaust system. However, make-up air fans do not entirely eliminate the problems associated with constant volume exhaust.
It has been suggested to vary the speed of an exhaust fan in proportion to the temperature of the air above the cooking units. While such an approach may reduce energy waste, it is not believed to be sufficient to ensure that all cooking by-products are removed, especially when large levels of cooking by-product are being generated while the cooking units are at a low heat condition. Thus, overexhausting may be reduced somewhat but with the risk of allowing cooking by-product to escape from the hood and into the kitchen. Escaping cooking by-product is to be strictly avoided.
It has also been suggested to vary the speed of the exhaust fan in accordance with the level of cooking by-product in the flow path. While this may reduce risk of underexhausting and may even help to minimize occasions of overexhausting, other drawbacks may present themselves. In particular, the sensors used to monitor for the by-product in the flow path are to be placed directly into the flow path of the by-products, which may permit grease, for example, to accumulate on the sensor risking damage thereto. In particular, such systems are not believed to be very effective in that the sensor may become coated or clogged with grease or other cooking by-products which may alter the ability of the sensor to accurately sense and respond correctly to the level of cooking by-products. Under these circumstances, the exhaust fan may be erroneously caused to operate at the wrong speed resulting in over- or underexhausting.